The perilous journey of Wyoming's migrating pronghorn

"All of my field work was for this one picture," says wildlife photographer Joe Riis, who set remote cameras along the Green River for over a month before he got this shot.

Joe Riis

Photographer Joe Riis and writer Emilene Ostlind spent four seasons trying to document the pronghorn migration to and from Grand Teton National Park. Here, one of Riis' first photos shows a group of pronghorn running through burned sagebrush on their way to the park.

Joe Riis

Each spring and fall, thousands of pronghorn cross the highway at Trappers Point six miles west of Pinedale, Wyo. Two rivers and the subdivision in the background of this photo pinch the migration corridor to just a half-mile in width. The Wyoming Department of Transportation funded a wildlife overpass at this intersection. Riis remembers hearing hooves tap the asphalt as he took this photo. "Next fall, they won't be doing this," he says. When the overpass construction is complete, cars will drive through a tunnel while the pronghorn run safely over a wide bridge covered with grass.

Joe Riis

The little buck at the right of this photo "probably hadn't crossed many fences before," says Riis. "This was its first fall migration." This wildlife-friendly fence near Trappers Point has a smooth bottom wire high enough above the ground for pronghorn to slip underneath, but even well-built fences can hinder migrating pronghorn, especially if snow piles up. As Riis snapped more photos, the young buck backed up and watched the older doe beside him duck under the fence before following her.

Joe Riis

Riis took this photo lying on his belly in the snow while 23 does bunched up against a fence along the highway at Trappers Point. Pronghorn are the only ungulates endemic to North America, and they've adapted to thrive on the sagebrush steppe. Their wide-set eyes provide a nearly 360-degree range of binocular vision, helping them spot threats in time to sprint away. They are resilient, but rely on wide-open spaces for survival.

Joe Riis

Ostlind scans the pronghorn summer range from Blacktail Butte in Grand Teton National Park. The pronghorn raise their fawns in the hayfields and sagebrush below. In the distance is the mouth of the drainage the migrating pronghorn follow into the Gros Ventre Mountains.

In 2007, an especially energetic and ferocious wolverine known as M3 was fitted with a GPS collar in Glacier National Park in Montana. Data from it showed the animal climbing a rocky cirque to summit 10,466-foot Mount Cleveland and rambling through a 700-square-mile home range. Another wolverine collared in Grand Teton National Park in 2009 traveled more than 500 miles to central Colorado over a couple of months, crossing the significant barriers of Interstates 80 and 70 along its way to Leadville. It was the first wolverine known to visit Colorado in 90 years. These aren't typical seasonal migrations, but they demonstrate an important point: that wild animals need room to roam between national forests, parks and other protected areas.

Pronghorn migrate for ecological reasons, seeking to avoid deep snow, find nutritious forage or return to places where they have already successfully given birth. But mysteries remain. Why does one pronghorn migrate a long distance while another makes a short journey or lingers year-round near its winter range? Why, in some populations, do more females than males migrate? And why would a pronghorn migrate one year and not the next? Andrew Jakes, a Ph.D. candidate from the University of Calgary who is studying the animals, says, "Migration is a complex behavior. We have the ability as scientists to tease apart that complexity." Many of the mechanisms remain unknown, but one thing is clear: "Migration is an adaptation. It's an important strategy to survive and reproduce, and it needs to be maintained."

Orange dots on a poster of mapped waypoints tell the story of one doe collared in Teton Park in October 2003. She migrated south to the Green River Basin with the rest of the herd for the winter. The following spring, rather than just following the corridor back to the summer range, she took off into the Gros Ventre high country, testing drainage after drainage. "From an anthropomorphic impression, it looked like she was trying to find a different route," says Steve Cain, lead biologist at Grand Teton National Park. When the alpine peaks at the top of the range stopped her, she quit exploring, returned to the regular migration corridor, and made her way up to the park for the rest of the summer.

Studies of the Teton population show "how tremendously fragile the corridor is," says Cain. "It's clear the existence of the population in Jackson Hole is completely dependent on maintenance of the corridor."

Today's research is painting a picture of a vibrant landscape, where wildlife flows like water across the boundaries between private and public, protected and working, Canadian and American landscapes. As one researcher puts it, "Us humans do not have a grasp on it yet."

As Jake and I drank our coffee, sandhill cranes hollered back and forth across snowmelt ponds. They strutted across the snow and grass searching for insects and then raised their long beaks to cry out, heads bobbing. The cranes seemed out of place here, like ballerinas on a mountaineering expedition. A yellowish coyote trotted into view, its pointy nose lowered, bushy tail swept behind. Every few steps, its paws broke through the snow, and it sank to its belly.

After the coffee and a short, sun-warmed nap, we left our packs and wandered along a dry ridge. On a small pond, two trumpeter swans glided away from us, climbing out of the water with their huge black feet splayed on the snow. Noisy ground squirrels scrambled to the tops of sagebrush to flick their tails and chatter their warnings. The clear zippering songs of sparrows rose from the sagebrush.

Because pronghorn are one of the bigger "sagebrush obligates"-- species adapted to thrive in a sagebrush ecosystem -- protecting their habitat benefits a lot of other animals, including some of the species Jake and I encountered that day in the mountains. One pronghorn wearing a satellite collar wandered across parts of Alberta, Saskatchewan and Montana during a single fall hunting season. The map that it generated helped launch an international agreement called the Northern Sagebrush Steppe Initiative, which coordinates conservation of sage grouse, mule deer and pronghorn. In western Wyoming, private-land conservation easements targeting pronghorn corridors also protect habitat for other native species, including sage grouse and mule deer. When wildlife photographer Joe Riis set remote cameras along the Path of the Pronghorn, he captured images of not just pronghorn, but also deer, elk, wolves, mountain lions and bears all using the same trails.

In late afternoon, Jake spotted a group of nearly 60 pronghorn carving a path through the snow up the valley toward us, making their way in single file. Whenever the leader -- usually a doe -- got tired of breaking trail, another would pass her and take the front of the line. In one place, they followed the track we had left through a pocket of snow.